1. Field of the Invention
The present invention relates to an image projection apparatus.
The image projection apparatus is used as a projector.
2. Description of the Related Art
In recent years, use of projectors has been widely spread.
Particularly, a short throw projector, which forms an image with a light flux emitted from an image display element on a screen through a refracting optical system and a concave mirror, has become highly popular.
In such a projector, an optical system including the refracting optical system and the concave mirror to form a projection image is a so-called projection optical system.
The above-described type of the projection optical system in the projector is generally made of the combination of a refracting optical system including a plurality of lenses which have an optical axis in common and a concave mirror having an aspheric surface or a free-curved surface.
Recently, a method to employ the free-curved lens in the refracting optical system in the projection optical system has been proposed (refer to Japanese Patent laid-open application publications Nos. 2008-225455, 2008-242025, 2008-242028, 2011-033738, 2011-150029, and 2011-150030).
When the free-curved lens is employed in the refracting optical system, a direction of a light beam can be controlled minutely by the optical function according to the position of the surface of the free-curved lens.
Therefore, distortion in a projection image can be reduced by employing the free-curved lens, the entire projection optical system can be downsized, and at the same time improved image quality can be achieved.
However, on the other hand, such a case includes the following problems.
That is, the projection optical system in which the free-curved lens is assembled properly as designed can validly realize the above-described effect; however, if it includes an assembly error, the function of forming a projection image is deteriorated significantly.
Therefore, the effect of the free-curved lens cannot be realized even if assembly error is only a slight because the lens surface is formed to be a free-curved surface.
In light of the above, assembly error causes significant deterioration in the image quality of the projection image.
One option to solve the above-described problem is to configure the free-curved lens to condense light weakly. Thus, the function of the free-curved lens is mainly employed for minute control to correct a distortion or a field curvature.
When the free-curved lens is configured to condense light weakly and is formed to have a refractive-angle difference between the incident light beam toward the free-curved lens and the emission light beam from the free-curved lens is small, the deterioration in the function of the free-curved lens caused by assembly error can be suppressed.
Thereby, the image degradation caused by assembly error of the free-curved lens can be reduced.
However, when the free-curved lens is configured to condense light weakly, the thickness of the lens is reduced. Consequently, an influence on the image caused by ghost light is apt to appear.
That is, when the light beam emitted from the image display element enters the free-curved lens and is emitted from the lens surface of the emission side, a part of the light beam reflects on the lens surface of the emission side.
The reflected light beam goes back to the lens surface of the incident side and reflects on the lens surface, and then the light beam is emitted from the lens surface of the emission side as ghost light in relation to a regular light beam.
Because the free-curved lens is configured to have a small refractive-angle difference between the incident light beam and the emission light beam, the optical path of the light beam which is to be the ghost light extends adjacent to the optical path of the regular light beam.
Thus, a ghost image is generated on the projection image.
The above-described related arts which propose projectors using the free-curved lens do not disclose or suggest a solution for such an influence caused by the ghost light.